What causes this activity(which you consider as life)?
The view I find to be most rational is it is caused by the underlying arrangement and interaction of the molecules/atoms/quarks/matter.
Absence of 'what', causes this 'absence of activity'(death according to you)?
Absence of the specific pattern and activity of those underlying molecules/atoms/quarks/matter.
You mean to say, 'life' and 'death' do not exist in reality but they exist only in our perception. Are they(life and death) like dreams?
Depends how you define reality, and what you consider it means when you say something is "real". I consider illusions to be real, for example, but that are merely different than what they appear to us to be. "Life" and "death" are descriptors for our interpretation of characteristics that certain things display (or do not display).
Are they like dreams? Certainly in the way that "dreaming" is another descriptor for certain activity. As are words like "motion".
Is that what you meant?
So, when a mass is moving, its property is 'moving' and when the mass is at rest, its property is 'rest'.
Instead you should have said 'inertia' is the property of a mass.
No, inertia is different, and not a descriptor of activity. If I had wanted to use inertia then I would have written it.
It requires a 'force' to change this state of inertia. Similarly 'life' is required to animate a cell.
Do you understand what inertia is? Force does not change a state of inertia. Force changes motion.
If force can exist, life also can exist. 'Force' is external to a mass. Similarly 'life' is external to a 'cell'.
Force is a descriptor and measure of mass * acceleration. It does not exist separate from those. It exists as a descriptor of activity, and as a relative measure of that activity: the acceleration of mass. It is not external to it.
Likewise life "exists" as a descriptor of activity. It is not external to it.
Is the 'motion' of a mass without any cause?
Generally no, but radioactive decay does not appear to have a cause, or no observable cause, and decay can throw out some particles with mass.
Otherwise all motion requires a cause.
So, how can the 'characteristics of life' be without any cause?
They're not... they are caused by the same things... the underlying motion and interaction of the particles etc, and it is only our interpretation of that pattern of activity that we call "life".